The disclosed subject matter relates generally to gas discharge lasers, and more particularly, to methods and systems for increasing service life of a gas discharge laser chamber.
Electric discharge gas lasers are well known for utilization is such fields as integrated circuit photolithography manufacturing processes as DUV light sources. The advent of immersion photolithography has required manufacturers of such laser systems to offer lasers that can produce 60 to 90 Watts of average power and more, meaning the laser light source needs to operate at higher pulse repetition rates, for example 6 kHz or more. In addition, for cost of operation reasons users of such light sources are demanding longer and longer laser chamber lives, usually measured in number of pulses until the chamber has to be replaced. One proposal for longer chamber live is to utilize at least one of the electrodes, between which an electric discharge excites a lasing gas gain medium to cause lasing, which can erode for a longer period of time and still adequately support such an electric discharge. This results in more debris and metal fluoride particles, formed by the reaction of electrode materials with the fluorine gas usually forming part of the lasing gas over the life of the laser chamber.
Several particle trap mechanisms (e.g., particle removers) are included inside the gas discharge laser chamber. A metal fluoride trap (“MFT”) has been used in such laser systems in the past as evidenced by U.S. Pat. Nos. 5,018,161 and 5,359,620, (i.e., '161 and '620 patents respectively) issued to Applicants' assignee Cymer, Inc., the disclosures of which are hereby incorporated by reference in their entirety for all purposes. The MFTs described in the '161 and '620 patents were primarily designed to generate a clean gas flow to and across the laser windows of the gas discharge laser chamber. The clean gas flow across the laser windows protects the windows from metal fluoride particle buildup. The MFTs described in the '161 and '620 patents were not designed for or intended to remove any significant portion of the particles present in the gas discharge laser chamber. The MFTs described in the '161 and '620 patents were especially not designed to extend service life of the gas discharge laser chamber of an order of magnitude or nearly so. In addition, particle traps have been used effectively, as evidenced by co-pending U.S. patent application Ser. No. 10/815,387, filed on Mar. 31, 2004, published on Oct. 13, 2005, Pub. No. 20050226301, the disclosure of which is hereby incorporated by reference in their entirety for all purposes. However such particle trap mechanisms are eventually saturated by the metal fluoride particles, leaving metal fluoride particles to circulate in the gas discharge laser chamber.
Lambda Physik utilizes a two stage filter as evidenced in U.S. Pat. No. 6,768,765, but without precipitation filters. GigaPhoton similarly uses an MFT of one stage which can be either a mesh filter or a precipitation filter as evidenced in U.S. Pat. Nos. 5,373,523 and 6,570,899. GigaPhoton Patent No. 7099365 utilizes a filter in a gas replenishment system connected to a laser chamber.
What is needed is to increase the service life of the gas discharge laser chamber is a higher particle trapping capacity of metal fluoride particles that can be removed from the gas flow. Applicants propose apparatus and methods to reduce the potentially very detrimental circulation of such metal fluoride particles in the gas discharge laser chamber, including reducing possible damage to the laser chamber windows and the possible reduction in discharge voltage required to produce a given pulse energy. These improvements can also provide a reduction in required gas circulation fan arc-free blower speed, especially at the elevated pulse repetition rates. Blower speed has limits bounded by vibration issues, power consumption limits and commensurate heat dissipation requirements.